1. Field of Invention
This invention relates to converting full color image information for dot next to dot (DND) printing on a highlight color printer. In particular, the invention relates to using complementary halftone screens in a way that generates highlight color images for input full color image information that preserves gradation and does not require complicated look-up tables.
2. Description of Related Art
Highlight color printers that print images comprising black dots and one other color of dots, e.g. red dots, are known, as described in U.S. Pat. Nos. 4,078,929, 5,223,857, and 5,260,799. Such highlight color printers are typically called K+1 printers, where K+1 represents the fact that the printer uses black [K] and one other color, typically red, to produce images. Since on such highlight color printers a dot can be black or red, but not both, the printing method is called dot next to dot (DND). When reproducing a full color image using a highlight color printer, the full color image must first be converted into highlight color images of black and red, i.e. K and R values representing the highlight color image, where K represents black and R represents red. This Kxe2x88x92R image must then be converted to a binary image, by halftoning, for printing. The binary Kxe2x88x92R image will be called the highlight color tint or simply the tint. The two step conversion is typically performed using a look-up table.
However, the look-up table approach is cumbersome, especially if gradation in the original image is to be preserved. For example, if 24-bit full color image information is provided, there are approximately 16 million different possible input values. Coarse quantization of the input can be used to reduce the size of the look-up table, but generating and implementing the look-up table, especially to satisfy the DND printing requirement, is not straightforward.
The invention provides a method and apparatus for converting full color image information to highlight color tints without using a complex look-up table.
The invention provides a method for generating highlight color tints for fall color image information that preserves gradation. According to the method, complementary halftone screens are applied to the highlight color image.
In one aspect of the invention, full color image information is converted to a highlight color printing space, such as the KR space, and converted pixel values in the printing space that are out of a printing gamut are mapped into the printing gamut before the halftone screens are applied.
In one aspect of the invention, full color information is transformed to the LCH color space. The H value for each pixel is dropped. If the resulting LC value is within the printing gamut, the LC value is transformed to KR values. If the LC value is not within the printing gamut, the LC value is mapped into the gamut and then transformed into KR values.
In one aspect of the invention, a printing gamut in the LC space is generated by mapping a line where K+R=Constant into the LC color space.
In one aspect of the invention, out-of-gamut LC values are mapped into the gamut by using a constant L value.
In one aspect of the invention, out-of-gamut LC values are mapped into the gamut by selecting a point within the gamut that lies on a line connecting a center point of the gamut on the L axis and the LC value.
The invention also provides an image forming apparatus that includes an image value transformer, an image value mapper and a halftone screen processor. Full color image information is converted to a highlight color printing space by the image value transformer. If the converted printing space value for a pixel is not within the printing gamut, the image value mapper maps the printing space value into the printing gamut to obtain a mapped printing space value. The halftone screen processor applies halftone screens to the mapped printing space value and other printing space values that are within the printing gamut to obtain highlight color tints.
In one aspect of the invention, the image forming apparatus also includes a controller for controlling the operation of the image value transformer, the image value mapper and the halftone screen processor.
In one aspect of the invention, the image forming apparatus also includes a memory for storing full color image information, highlight color image information, and highlight color tints.
In one aspect of the invention, the image forming apparatus also includes a display that produces an image based on the highlight color tints.
In one aspect of the invention, the image value transformer converts fill color image information into the LCH color space. If the LC value for a pixel is not in a printing gamut, the image value mapper maps the out-of-gamut LC value into the printing gamut. The mapped LC value and other LC values within the printing gamut are converted to a printing space. The halftone screen processor applies complementary halftone screens to the converted values in the printing space to obtain highlight color tints.